The present invention concerns firmly fixed decorative ceramic color layers applied to glass or glass ceramic substrates.
Ceramic colors are often used to decorate glass and glass ceramics. They usually consist of one or several vitreous substances (base enamel or vitrification) and an admixture of one or several pigments (coloring agents). Colored inorganic oxidic compounds, e.g. spinel structures, may be used as pigments. The pigments may thereby usually not be corroded or only slightly corroded by the vitrification (dispersion coloring).
Ceramic colors are usually manufactured by mixing frits (quenched vitrifications) with the inorganic pigments. The mixing may thereby already occur during the melting process of the vitrification (fritting) or before as well as during a subsequent pulverization process of the quenched glass mass. The ceramic colors ready for use are then provided in the form of powdery mixtures of coloring agents and glass. The coloring agents consisting of metal oxides are soluble in the glass melts to a certain extent. The solubility of the coloring agents usually increases with the temperature of the glass melt.
To decorate glass or glass ceramic objects, which are usually present in finished form, the color powder is applied to the substrate to be coated in a number of ways. The powder may be present in particles with average diameters of between less 1 .mu.m and 40 .mu.m.
To adhere the ceramic colors before stoving, they are dispersed in highly fluid to viscous application agents, which evaporate partly during drying of the applied colors and partly during stoving. Examples of this process are mixing with screen printing oils, followed by direct or indirect screen printing (metachromotype process) or mixing with thermoplastic material and subsequent screen printing under the influence of heat. If the ceramic colors are mixed with suitable organic substances, they may be sprayed onto the substrate (spray colors).
To preserve the durability in use of the objects to be decorated, the layers applied to the substrate must be fused with the substrate by means of stoving, so as to form a solid unit. During stoving, the glass powder melts and forms a compact layer in which the coloring agent particles are firmly embedded.
If the substrate to be coated is a solid-state body, the stoving of the decoration is usually performed simultaneously with glass cooling or during glass prestressing. If the substrate is a glass ceramic, the stoving of the decoration is usually performed during the ceramization process, i.e. the decoration is applied to the green glass and stoved during ceramization. However, since the objects to be coated are already present in finished form, this means that the ceramic colors may only be stoved at temperatures at which no deformation of the pre-formed substrates can occur, i.e. the liquidity temperature of the base enamel of the ceramic color must be lower than or, at most, equal to the upper cooling temperature (annealing point) of the glass substrate. The annealing point of borosilicate glass and soda lime glass is approximately 570.degree. C. To avoid deformations of the substrate during stoving of the base enamel, ceramic colors which are to be suitable for these glasses must fluidize at temperatures below 600.degree. C., i.e. at a temperature of 600.degree. C. the base enamel must display a viscosity below app. 10.sup.2 poise. Typical temperatures for ceramization are in the range of app. 800-950.degree. C., so that hereby base enamels with a higher liquidity temperature may be used.
These requirements regarding the properties of the substrate and the base enamel to be used result in a further problem based on the different thermal expansion coefficients of the substrate and the coating material. If the thermal expansion. coefficients of the ceramic color and the pre-formed glass or glass ceramic body deviate from each other, tensions occur in the cooled layer body. If the stoved layer has a larger thermal expansion coefficient than the coated substrate, tensile stress results in the coating material. If, however, it has a lower thermal expansion coefficient than the substrate, it is subject to compressive tensions. The scale of these tensions depends on the difference of the thermal expansion coefficients. Borosilicate glass has a thermal expansion coefficient of .alpha.=3.times.10.sup.-6 /K, whereas the thermal expansion coefficient of glass ceramic is .alpha.=.+-.-0.2.times.10.sup.-6 /K.
Lead as well as lead borate glass, which is commercially available in many varieties and at economic prices and displays the required low liquidity temperatures below 600.degree. C., is a typical base material (base enamel) for ceramic colors. Lead resp. lead borate glass, however, has thermal expansion coefficients in the range of app. 6.times.10.sup.-6 /K to 12.times.10.sup.-6 /K. The great divergence of the thermal expansion coefficients means that enormous tensile stress is present in the glass thus coated after cooling, so that cracks result, whose fissures may extend into the substrate material. The resilience to temperature differences and resistance to sudden change in temperature is also reduced in objects coated in such manner. According to the prior art, these disadvantages can only be alleviated in that the thermal expansion coefficients of the ceramic colors are adapted to the expansion coefficients of the substrates to be coated. This, however, would require ceramic colors with a high melting point, which again entails the danger that the glass or glass ceramic substrates already preformed become subject to deformation. After a certain period of time, the tensions generated by such maladjustment of the thermal expansion coefficients, however, lead to the color layer chipping off, if the maximum load of the composite arrangement is exceeded, whereby the color layer then falls off the objects resulting in chipping the object. According to the prior art, this can only be prevented in that a very thin colour layer is stoved onto the substrate, in order to keep the resultant tensions after stoving below the maximum lead. This, however, also means that the color effect (covering power, color impression) is partly very limited A further attendant disadvantage is the fact that, during stoving, such thin color layers tend to form individual domains on the substrate to be coated, so that the surface is as a whole substantially rougher than in the case of an even color coat. As a result, important working properties such as chemical resistance to acids or alkalis during cleaning or abrasive behavior are considerably impaired. Likewise, the radiance of rough surfaces is substantially lower than that of even surfaces. A further disadvantage which rendered the coating of, in particular borosilicate, glass with colors of, in particular non-lead, base enamel difficult was the fact that such non-lead colors are very brittle, due to their lower elasticity compared to lead base enamels, and therefore displayed reduced adhesion to the substrate and did not permit application of the layer thickness required to generate the attractive color impression of a decorative layer to be created with the ceramic colors.